The present invention relates to an optical communication system and more particularly to a system for monitoring an optical communication system.
In long distance fiber optic communication systems it is important to monitor the health of the system. For example, monitoring can be used to detect faults or breaks in the fiber optic cable, faulty repeaters or amplifiers or other problems with the system.
Prior art monitoring techniques include the use of a testing system which generates a monitoring signal and modulating the monitoring signal onto a single channel (or wavelength) with the transmitted data signal. For example, the data signal may be amplitude modulated by the monitoring signal. A loop-back coupler within an optical amplifier pair or repeater located downstream is used to return a portion of the transmitted signal (data signal plus monitoring signal modulation) to the testing system. The testing system then separates the monitoring signal from the data signal and processes the monitoring signal to examine the health of the transmission system. U.S. Pat. Nos. 4,586,186 and 4,633,464 C. Anderson et al. disclose a similar technique to modulate monitoring response information from a repeater onto the main data signal to monitor the health of the system.
Optical time domain reflectometry (OTDR) is another technique used to remotely detect faults in optical communication systems. In OTDR, an optical pulse is launched into an optical fiber and backscattered signals returning to the launch end are monitored. In the event that there are discontinuities such as faults or splices in the fiber, the amount of backscattering generally changes and such change is detected in the monitored signals. Since backscattering and reflection also occur from elements such as couplers, the monitored signals are usually compared with a reference record, new peaks and other changes in the monitored signal level being indicative of changes in the fiber path, normally indicating a fault. The time between pulse launch and receipt of a backscattered signal is proportional to the distance along the fiber to the source of the backscattering, thus allowing the fault to be located. In a wavelength division multiplexing (WDM) system, one wavelength is usually assigned as the OTDR channel.
Typically, line monitoring equipment (LME) detecting a returned portion of the transmission signal is employed when the transmission system is in-service and OTDR is employed when the system is out-of-service. Therefore, crosstalk between the OTDR channel and the data channels is not a concern. Since the line monitoring equipment is used in-service, however, crosstalk is a concern in this case. Specifically, crosstalk arises between the returning portion of the signal and the data channels traveling on the opposite-going transmission path.
Accordingly, it would be desirable to provide line monitoring equipment for an optical transmission system that reduces crosstalk between the data channels and the returning portion of the signal that is to be monitored.
In accordance with the present invention, an optical communication system is provided that includes first and second optical transmitters/receivers remotely located with respect to one another and which are coupled together by first and second optical transmission paths for bidirectionally transmitting optical information therebetween. First and second optical amplifiers are respectively disposed in the first and second optical transmission paths. At least one loop-back path optically couples a portion of a WDM optical signal from the first to the second transmission path. The loop-back path includes a filter for transmitting a monitoring channel but not a data channel included in the optical signal portion traversing the loop-back path.
The loop-back path may include first and second optical couplers disposed in the first and second transmission paths, respectively. In one particular embodiment of the invention, the transmission system also includes an OTDR path for coupling a backscattered signal from the first to the second transmission path. In this embodiment, the loopback path and the OTDR path overlap at least in part and the first and second couplers further couple the backscattered signal from the first to the second transmission path.
In another embodiment of the invention, the WDM optical signal includes a plurality of data channels that occupy a given data bandwidth and the monitoring channel is located at a wavelength outside of the given data bandwidth. In some cases the monitoring channel is located at a wavelength below the given data bandwidth, or alternatively, above the given data bandwidth.
In accordance with another aspect of the invention, a method is provided for monitoring an optical communication system that includes first and second optical transmission paths coupling a first transmitter/receiver to a second transmitter/receiver for bidirectionally transmitting optical information therebetween. The first and second optical transmission paths respectively include first and second optical amplifiers. In accordance with the method, a WDM signal is first transmitted. The WDM signal includes a monitoring channel and at least one data channel through the first optical transmission path. Next, a portion of the WDM optical signal is coupled from the first transmission path and filtered so that the monitoring channel but not the data channel is transmitted. The filtered portion of the WDM optical signal is coupled to the second transmission path. Finally, the monitoring channel is detected to access the status of the transmission system.